Passive transpiration cooling system



Dec. 1, 1964 M. J. BRUNNER 3,159,012

PASSIVE TRANSPIRATION coouua SYSTEM Filed Nov. 25, 1960 NNN'K NN ATTORNEY United States Patent 3,159,012 PASEVE TRANSPERATIGN (IOQLTNG SYEi'IEld Mathias John Brunner, Broomall, P2,, assignor to General Electric Company, a corporation of New York Filed Nov. 25, 1M0, Ser. No. 71,617 13 Claims. (Cl. 622-467) This invention relates generally to heat resistant structures and more particularly to a heat resistant structure incorporating a transpiration cooling system.

One problem which has arisen in the field of missile research is the protection of missiles from the intense heat generated when a missile re-enters the atmosphere of the earth. In the past, a solution which has been utilized is to construct the missile body with walls of sufficient thickness so that despite the erosion or ablation of these walls, the necessary structural strength remains to complete the re-entry intact. This ablation is not necessarily uniform, however, and furthermore even a uniform abla- .tion necessarily causes a change in the aerodynamic performance of the missile. An example of this would be a loss in lifting surface area of a wing or fin element.

Current engineering endeavor is being directed towards maintaining the initial configuration of the surface of the missile. One result of this research is transpiration cooling, which has been found to be a suitable method for preventing the excessive ablation of the missile surfaces so that deformation of these surfaces is substantially elimi-nated.

Previous transpiration systems have utilized a liquid, such as water, which is pumped through pores in the outer skin of the missile, vaporized and carried away. Since the heat which the missile generates varies in these systems, a greater flow of water is required as a greater heat is generated. A regulating system must therefore be incorporated to vary the flow of water in accordance with the cooling requirements. An alternative method of obtaining the necessary cooling would be to continuously pump water at a rate sufficient to supply the maximum cooling requirement anticipated. A system utilizing the latter method would obviously be was-ting some of the water. A more desirable solution would be a system which was regulated so that coolant would not be needlessly wasted and which nevertheless did not require a complicated regulating means.

It is therefore an object of this invention to provide a transpiration cooling system which is self-regulating.

It is also an object of this invention to provide a transpiration cooling system in which coolant is used up only as needed.

It is a further object of this invention to provide a heatresistant structure which will not substantially deform when subjected to heating.

It is an additional object of this invention to provide a heat-resistant structure which is light in weight and which requires no auxiliary regulating apparatus.

Briefly stated, in carrying out the invention in one form which may be preferred, a porous metal sheet member is provided to act as the outer shell of the heat-resistant structure. Against the inner surface of this sheet member a solid cooling material is positioned. This cooling material is one which will change in form when it is heated in such a manner that it will absorb heat, become fluid adjacent to the sheet member, pass through the pores in the sheet member, and finally be carried away. The solid coolant material will preferably become sufliciently plastic in nature upon heating so that it will continue to maintain substantially continuous contact with the inner surface of the porous sheet member as the liquid portions of the material are carried away, and in many cases will vaporize.

3,159,912 Patented Dec. 1, 1964 ice The invention will be better understood from the following description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic cross-section of a heat-resistant structure according to the invention;

FIGURE 2 is a schematic cross-section of a portion of a probe incorporating the invention; and

FIGURE 3 is a schematic cross-section of a portion of a missile shell incorporating the invention.

Referring now to FIGURE 1 of the drawing, in accordance with the invention, a thin sheet member l, which may be of metal, is provided overlying a layer 2 of a solid coolant material. The outer surface 3 of the sheet member 1 is intended to be that part of the heat-resistant structure which is exposed ot the effects of heat. If the heatresistant structure were utilized on a missile therefore, the outer surface 3 will become heated as a result of the rapid movement of the missile through the atmosphere. On the other hand, it will be evident that the outer surface 3 will be equally suitable if the heat should be from some other source. The heat on the outer surface 3 will be conducted through the sheet member 1 to its inner surface 4. The solid coolant material 2 adjacent to and prefenably in intimate contact with the inner surface 4 will thereby become heated. This solid coolant will preferably melt upon heating although in some instances a coolant which sublimates can be utilized. It is evident that the heat of fusion (or vaporization in the case of a subliniating material) of the coolant material will therefore be absorbed providing cooling for the structure.

The sheet member 1 is provided with a plurality of apertures or pores 5. The liquid or gaseous form of coolant material 2 passes through pores 5, at the same time absorbing further heat. become gaseous before reaching the outer surface 3 of the sheet member and the gases evolved will become further heated thereby providing further cooling for the structure.

As a result of the motion of a missile incorporating the heat-resistant structure of this invention, the gases which have passedthrough pores 5 to outer surface 3 will pass generally rearwardly over the outer surface of the missile. These gases protect the outer surface 3 since they form a heat insulating barrier.

Sheet member 1 has already been described as being thin and having pores. These characteristics can be obtained by forming apertures in a sheet member which may be a metal or a non-metal, for example, a ceramic. Another alternative would be utilizing a metal mesh for sheet member 1. When used for missile purposes, sheet member 1 should be chosen to be as thin as possible commensurate with strength requirements, so as to keep its weight as low as possible. Thicknesses from 0.020 to 0.050" "have been found suitable for use in some applications. Pore sizes and numbers are restricted first of all by the strength requirements of sheet member 1. In addition, it has been discovered that certain coolant materials will be extruded through the pores if the pores are made too large. On the other hand, the pores mus-t be densely enough spaced so that substantially all the coolant can escape through the pores and not become trapped.

As was previously brought out, some coolant materials will melt while others will sublimate. It will be evident to those skilled in the art that a primary requirement for a coolant is that in changing from its solid form to a gaseous state it should absorb great quantities of heat. In addition, experimentation has indicated that a solid coolant which softens or becomes plastic in nature upon heating is desirable. This factor can be explained by pointing out that only by the efiicient transference of the heat from sheet member 1 to solid coolant 2 will sheet member 1 be effectively cooled. This heat transference is greatly fa- The coolants in liquid form may ciliated by having intimate physical contact between solid coolant 2 and inner surface 4 of the sheet member. As the solid coolant melts away near inner surface 4 and passes through pores 5, the remaining solid coolant should in turn come into intimate contact with, and conform to, the configuration of inner surface 4. It may be found vpreferable to utilize some type of biasing means such as a spring to force solid coolant 2 against .inner surface d. Another possible biasing means would be the pressure of the gases evolved from the solid coolant. In other appllcations, the time when cooling is required may be so short or the over-all heat absorption so small that no biasing means is needed since the solid coolant initially in contact with inner surface 4 would supply sufiicient cooling.

One factor which may be important in the selection of a solid coolant derives from this requirement of good physical contact. Some solid coolants, when heated, do not change completely into a liquid or gas, but a solid residue or char remains. It will be seen that a residue or char might therefore provide a heat insulating barrier between solid coolant 2 and inner surface 4 and in some cases might clog pores 5, so that a coolant which chars would be unsuitable for some purposes.

While the changes in form of the cool-ant from a solid to a gas have been described as melting and vaporization, which may be considered physical changes, it should be clear that pyrolysis or chemical decomposition is another means for absorbing heat. To illustrate, the depolymerizing of some polymers has been found to be a desirable method of heat absorption, particularly when great quantitles of hydrogen are generated.

Some materials which have been found suitable for use as solid coolants ar nylon, polyethylene, and polystyrene. Ammonium chloride has also been used successfully. This compound sublimates, and while it has been effective in some applications, it may not be the best material for all purposes. Its coolant qualities are improved when it is mixed with polyethylene. The coolant materials given above are not intended to be all inclusive, but merely exemplary.

One desirable aspect of the cooling which results from the disclosed invention is that -it is self-regulating. It is only when the temperature reaches the point where the physical strength of the structure is about to be impaired, ie, its temperature of deformation, that cooling is necessary, and by proper selection of a solid coolant the trans piration of the coolant will not occur until this point is reached. Moreover, as more and more heat is generated, the transpiration system automatically responds by giving off greater quantities of gas and absorbing more heat. The self-regulating yet passive nature of this transpiration system makes its'use highly desirablesincethe weight and .possible breakdown of an active type of regulating system involving valves or the like can'be avoided.

A second desirable aspect of this system is that it is the solid coolant which is consumed rather than the sheet member which formsthe outer shell of the missile or other object to be cooled. Because of this, a thin and therefore light-weight missile shell is possible; and the surface con- 'figurationof the missile which is most efficient .will be maintained throughout the flight of the missile.

While it should be evident that this invention is primarily expositive ofthe broad idea of passive transpiration cooling, specific examples of how this invention can be utilized will now be given.

Referring now to FIGURE 2, a portion of a probe member 6 which may be mounted on a missile is shown. Probe member 6 is provided with a porous sheet member 7 covering its leading edge. A single block 8 of a solid coolant such as heretofore described is biased against the inner surface of sheet member 7 byspring 9 which reacts against plate 10 secured to the probe interior. In operation, the leading edge of probe 6 is expected to become intensely heated while the sides of the probe are heated to a lesser degree; therefore, the transpiration cooling necessary biasing.

apparatus according to the invention is represented as applied to this leading surface. In a similar manner, other parts or areas which experience intense localized heating may be expeditiously cooled by providing a block of the solid coolant at these positions with suitable porous sheet members and associated biasing means.

Where a larger area experiencing varying degrees of heating or having a more complex surface configuration is desired to be cooled, it may be preferable to provide a plurality of blocks of solid coolant with the blocks in areas expected to be most highly heated having a greater thickness than those intended to cool areas less intensely heated.

Referring to FIGURE 3, a thin, porous sheet member 20 is represented as a portion of the outer shell of a missile. Immediately behind this shell a number of blocks of coolant material 21, Hand 23 are positioned so as to be in contact with the inner surface 24 of sheet member Zll. The blocks 21 are shown as being thicker than the other blocks since they are positioned at a place expected to be more intensely heated. Block'22 is somewhat thinner since it is intended to cool a portion of the sheet member 2% which may be somewhat less heated. In a similar manner, thinner blocks 23 are positioned to provide the smaller cooling requirements of those portions of the shell which are relatively slightly heated.

As was previously described, suitable biasing means may be provided to maintain the coolant material in contact with inner surface of sheet member 24. This biasing means can be made so as to react against inner shell 25 of this portion of the missile. As is shown in FIGURE 3, a plurality of springs 26 may be used to provide the A separate spring 26 for each block of coolant 21, 22 or 23 is desirable since the rate at which these blocks may melt or sublime may vary and the individual springs will insure adequate biasing.

Measurements have been made upon specific examples of heat resistant structures according to my invention. In these tests, radiant heat was applied to the exposed metal surface (corresponding to 3 of FIGURE 1) by electrically heated radiators; and blocks of the heat absorbing material (corresponding to 2 of FIGURE 1) approximately two inches thick were pressed against the other surface of the metal (corresponding to 4 of FIG- URE 1). The tests were continued until the heat absorbing material was nearly all consumed, so that the length of the test varied markedly with the heat flux rate, from approximately a quarter of a minute for the highest rates to ten minutes for the lowest. In one series of tests, a sheet of molybdenum 0,050" thick, perforated with As-inch diameter holes occupying 25 percent of its surface, corresponded to 1 of FIGURE 1. Polystyrene blocks corresponded to 2 of FIGURE 1. Heat removal rates from 3.1 B.t.u. per second per square foot (at which rate the equilibrium temperature of the molybdenum was approximately 740 degrees Fahrenheit) to 10 -B.t.u. ,per second per square foot were obtained. This was with only sufficient air flow to carry the fumes through an exhaust hood and out of the laboratory. In another test, a 0033" thick sheet of tantalum, backed by polyethylene, Was subjected to a heat flux rate of 990 B.t.u. per second per square foot, at which rate the tantalum had an equilibrium temperature of approximately 3100 degrees Fahrenbelt. The apertures in the tantalum sheet occupied 13 percent of its surface, and were not simple round holes as in the case of the molybdenum, but were pierced in a fashion similar to a kitchen vegetable grater.

While the heat-resistant structure of this invention has been described particularly as to its use in missile technology, it should be evident that the passive transpiration cooling arrangement disclosed herein is also applicable to cooling any other device to which this structure can be adapted.

While a particular embodiment of a heat-resistant structure incorporating this invention has been shown and described, it will be obvious that changes or modifications may be made without departing from the invention. The concluding claims are intended to cover all such changes and modifications as fall within the true scope and spirit of the invention.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:

1. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing in form to pass through the pores of said sheet upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

2. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant which will evolve hydrogen by pyrolysis biased against said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing in form to pass through the pores of said sheet upon heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

3. A heat-resistant structure comprisin a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing to a liquid to pass through the pores of said sheet upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

4. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing to a gas to pass through the pores of said sheet upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

5. A heat-resistant structure as defined in claim 4 wherein said change to a gas includes a liquid state of said initially solid coolant.

6. A heat-resistant structure as defined in claim 5 wherein said change to a gas further includes an initial softening of said solid coolant whereby said solid coolant wil conform to the configuration of said inner surface, and a liquid state of said solid coolant. l

7. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant Which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing first to a liquid and then to a gas upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet and with the change from a liquid to a gas occurring in the pores of said porous sheet.

8. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a plurality of blocks of a solid coolant which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, spring means biasing each of said blocks against said inner surface, said solid coolant being comprised of a material capable of changing in form to pass through the pores of said sheet upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

9. A heat-resistant structure comprising a metal mesh sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant which will elvolve hydrogen by pyrolysis in contact with said inner surface of said metal mesh sheet, said solid coolant being comprised of a material capable of changing in form to pass through the mesh of said sheet upon the heating of said metal mesh sheet to a temperature lower than the temperature of deformation of said metal mesh sheet.

10. A heat-resistant structure comprising a porous sheet having its outer surface adapted to be exposed to the atmosphere, an initially solid coolant in contact with the inner surface of said porous sheet, said solid coolant depolymerizing upon the heating of said porous sheet due to said heating, said depolymerizing being a result of the absorption of heat from said porous sheet whereby said porous sheet is cooled.

11. A heat-resistant structure comprising a porous sheet having its outer surface adapted to be exposed to the atmosphere, an initially solid coolant in contact with the inner surface of said porous sheet, said solid coolant pyrolizing upon the heating of said porous sheet due to said heating, said pyrolizing being a result of the absorption of heat transferred from said porous sheet whereby said porous sheet is cooled.

12. A heat-resistant structure comprising a porous sheet having an outer surface adapted to be exposed to the effects of heat and an inner surface, and a solid coolant in contact with said inner surface of said porous sheet, said solid coolant being selected from the group consisting of nylon, polyethylene, polystyrene and ammonium chloride.

13. A heat-resistant structure comprising a metal sheet having an outer surface adapted to be exposed to the.

effects of heat and an inner surface, said metal sheet having a plurality of pores connecting said inner and outer surfaces, and a solid coolant which will evolve hydrogen by pyrolysis in contact with said inner surface of said porous sheet, said solid coolant being comprised of a material capable of changing in form to pass through the pores of said sheet upon the heating of said porous sheet to a temperature lower than the temperature of deformation of said porous sheet.

References Cited by the Examiner UNITED STATES PATENTS 1,894,775 1/33 Levenson 624 2,096,539 10/37 Gebauer 62-384 2,279,774 4/42 Bolton 117-132 2,335,930 12/43 Freeland 117-132 2,629,907 3/53 Hugger 117-132 2,688,563 9/54 Kiefr'er 117-132 2,697,058 12/54 Lasak 117-132 2,731,432 1/56 Toulmin 117-132 2,737,461 3/56 Heisler 117-132 2,941,759 6/60 Rice 62-239 2,946,702 7/60 Bach 117-132 2,952,561 9/60 Young 117-132 2,959,938 1l/60 Giardini 62-384 3,011,989 7 12/61 Russell 117-132 3,016,309 1/62 Roeser 117-132 ROBERT A. OLEARY, Primary Examiner.

WILLIAM J. WYE, Examiner. 

1. A HEAT-RESISTANT STRUCTURE COMPRISING A POROUS SHEET HAVING AN OUTER SURFACE ADAPTED TO BE EXPOSED TO THE EFFECTS OF HEAT AND AN INNER SURFACE, AND A SOLID COOLANT WHICH WILL EVOLVE HYDROGEN BY PYROLYSIS IN CONTACT WITH SAID INNER SURFACE OF SAID POROUS SHEET, SAID SOLID COOLANT BEING COMPRISED OF A MATERIAL CAPABLE OF CHANGING IN FORM 